Molecular probes intends to synthesize and spectrally characterize a large variety of long-wavelength, predominantly cationic dyes for evaluation as possible photosensitizers in photochemotherapy. Selective accumulation of cationic dyes in carcinoma cells has previously been demonstrated. Screening of all new synthetic dyes in vitro for their toxicity with and without illumination will be performed in a collaboration with Dr. Allan Oseroff of New England Medical Center. The most effective new dyes will be further evaluated in vivo for their dark toxicity, kinetics of loading, and mechanism and extent of phototoxicity. Results of this collaboration will be used to guide synthetic efforts for improved photosensitizers with greater tumor selectivity. higher accumulation in carcinoma cells, better retention and reduced toxicity from dyes currently used in photodynamic therapy such as hematoporphyrin derivative.